This invention relates to semiconductor devices such as light-emitting diodes (LEDs) and transistors, particularly to those employing nitrides or nitride-based compounds as semiconductors, and to a method of making such semiconductor devices.
Nitride-based semiconductor devices are capable of fabrication on substrates of either sapphire, silicon carbide, or silicon. Silicon in particular offers the advantages of being less expensive and easier of cutting than sapphire or silicon carbide. Unlike sapphire, moreover, silicon is electrically conductive and so lends itself to use as a current path. Offsetting these advantages of the silicon substrate is a relatively great voltage drop caused by the potential barrier between the silicon substrate and the nitride semiconductor. The nitride semiconductor LEDs with the silicon substrate have therefore required a drive voltage that is high enough to overcome the voltage drop.
Japanese Unexamined Paten Publication No. 2002-208729 teaches an LED configuration designed to preclude the noted shortcoming of the silicon substrate. It employs an n-type silicon substrate on which there is grown by epitaxy the main semiconductor region of the LED via a buffer region. The buffer region comprises an aluminum nitride (AlN) layer directly overlying the n-type silicon substrate, and an indium gallium nitride (InGaN) layer of the same n conductivity type as the silicon substrate on the AlN layer. The main semiconductor region comprises a lower cladding or confining layer of n-type gallium nitride (GaN), an active layer of InGaN, and an upper cladding or confining layer of p-type GaN.
In the course of the epitaxial growth of the successive layers of the buffer region and main semiconductor region of the prior art LED on the n-type silicon substrate, there occurs a partial diffusion of the aluminum of the lowermost AlN buffer layer, and of the indium and gallium of the overlying InGaN buffer layer, into the silicon substrate. The result is the creation, at the interface between silicon substrate and AlN layer, of a layer of the alloys or compounds of gallium, indium, aluminum and silicon. This alloy layer, as it might be so called, is in itself advantageous from the standpoint of LED efficiency as it reduces the potential barrier of the heterojunction between silicon substrate and AlN layer and so enables the LED to operate with a lower drive voltage than in the presence of the potential barrier. The LED is thus made less in power loss and higher in efficiency.
The trouble, however, is that aluminum, indium and gallium diffuse deeper down into the silicon substrate from the alloy layer. These Group III elements represent p-type impurities in the n-type silicon substrate, so that a pn junction was conventionally created in the substrate under the alloy layer. This applicant has ascertained that the pn junction caused a forward voltage drop of 0.6 volt or so. The residual potential barrier between the silicon substrate and the nitride semiconductor layers thereon was still so high that the voltage drop across this prior art LED (in other words, its drive voltage) was approximately 1.2 times as great as that across the sapphire-substrate LED. This shortcoming of the n-type silicon substrate in conjunction with the nitride semiconductor layers grown thereon manifests itself not only with LEDs but with transistors and other types of semiconductor devices of comparable make.
Another problem with LEDs concerns the electrodes that meet the dual, sometimes contradictory, requirements of enabling the emission of light therethrough and providing electric connections. A typical conventional solution was the creation of a transparent electrode, as of a mixture of indium oxide and tin oxide, on the light-emitting surface of the semiconductor chip, and on this transparent electrode, a metal-made bonding pad for connection of wire. The metal material of the bonding pad is easy to diffuse into the transparent electrode and even into the semiconductor region, the transparent electrode being as thin as, say, ten nanometers. A Schottkey barrier was therefore easy to be formed between the bonding pad and the semiconductor region. Blocking the forward current of the LED, the Schottkey barrier reduced current flow under the bonding pad and added to current flow through the outer parts of the semiconductor region.
As the n-type silicon substrate necessitates as aforesaid the application of a high drive voltage forwardly of the LED, both silicon substrate and semiconductor region cause greater power losses and generate more heat. The noted Schottkey barrier deteriorates as a consequence, permitting greater current flow therethrough and, in turn, causing reduction of current flow through the outer parts of the semiconductor region. The greater amount of light generated at the central part of the semiconductor region hardly leads to a greater amount of light actually emitted from the LED, because the bonding pad is impermeable to the light. Furthermore, with the reduction of current flow through the outer parts of the semiconductor region, a correspondingly less amount of light is generated and emitted through the transparent electrode. The prior art LEDs having nitride semiconductors on n-type silicon substrates were therefore mostly unsatisfactory in the efficiency of light emission.
It has been known to provide a current blocking layer of electrically insulating material between the bonding pad and the semiconductor region. This solution is objectionable because of the additional manufacturing steps, and consequent higher manufacturing costs, needed for fabrication of the current blocking layer.